1. Field of the Invention
This invention is in the field of vibration isolators and more particularly the invention relates to a special self centering low friction mounting for a vibration isolator.
2. Description of the Prior Art
Vibration isolators are well known in the art. They include means to attach one end of the isolator to a first member which may vibrate, a damper and spring arrangement intended to absorb the vibrations of the first member and means to attach the other end of the isolator to a second member which is desired not to vibrate. Vibration isolator mounts are used when it is necessary to reduce the dynamic loads transmitted from the vibrating member, or source, to vibration sensitive of fragile equipment mounted to or near the source. For most isolator mounts, it can be stated that the performance is greatest if the natural (or break) frequency of the isolator, when combined with the isolated mass, is as low as possible as determined from the equation f.sub.n =1/2.pi.(K/M).sup.1/2 where f.sub.n is the natural frequency, K is the stiffness of the isolator mount and M is the mass to be isolated.
One of the difficulties with designing and implementing a soft mount is that they sag tremendously as their break frequency (stiffness) is reduced. Mechanical spring isolator mounts are limited by their own structural integrity and stability when trying to achieve extremely low break frequencies. Viscoelastic isolator mounts also suffer from structural integrity problems when designed to break at low frequencies. One mount that seems ideally suited for overcoming the structural problems that come with low break frequencies is a pneumatic isolator system.
Using the advantages of compressible air as a spring to soften the interface between vibration source and vibration sensitive equipment is not new. Large trucks use an airbag type of pneumatic suspension to soften the ride for their payload. Optical benches are isolated from lab floors by way of soft pneumatic mounts. Large loads can be supported by these systems by air pressure and support piston surface area. The pneumatic spring rate is dependent on air pressure, air volume and piston area supporting the load. Therefore, with proper pneumatic isolator design one can support large loads with a soft mount.
One of the problems with the present day pneumatic isolator is its inability to support side loading, particularly due to is own weight. Present day pneumatic isolators operate exclusively in parallel with the gravity vector. This makes it very difficult to take advantage of the pneumatic suspension in a 6 degree of freedom isolation system where the isolator elements are angled, as when mounted in the familiar hexapod (Stewart Platform) or octapod arrangement. Many clever side support systems have been devised but never to the degree necessary to provide soft air support in all degrees of freedom. In the above mentioned copending application, ball bearings which are positioned in grooves provides the means for almost friction free movement of the piston while providing full support and alignment when the support is mounted substantially horizontally. When horizontal, the balls are self centering, that is, they tend to move back to the middle of the grooves as they are being used. While at a full stroke, the balls may hit a groove end causing momentary stiction, this position is short lived as the piston moves back and fourth. However, when the support is mounted at an angle, and because of a small amount of clearance necessary to maintain a free rolling assembly, the effect of gravity may cause the balls to work their way down hill to the bottom of the track and the self centering action, described above, when the support is horizontal, is lost.